At the end of this module, you will be able to:
- Define a "bleaching sequence"
- Describe the goals of bleaching and how to achieve a good bleaching sequence
- Describe how to represent bleaching stages and sequences
- Identify and define typical bleaching sequences
- Describe advantages or disadvantages of typical bleaching sequences
- Identify reasons for filtrate recycling
- Identify and describe filtrate recycling configurations
The following key questions are answered in this module:
What is a bleaching sequence?
The bleaching chemicals and the order in which they are used make up a bleaching sequence.
How can the chemical demand for bleaching be reduced?
Pulp to a lower kappa number and/or use an oxygen delignification stage prior to bleaching so there is less lignin to remove.
What do the letters in a bleaching sequence mean?
Each letter represents a different chemical. Chlorine Dioxide (D), Oxygen (O), Hydrogen Peroxide (P), Ozone (Z), and Alkaline Extraction (E)
What does ECF and TCF mean?
Chlorine used to be the major pulp bleaching chemical, but it is no longer used because is harmful to people and the environment. ECF stands for elemental chlorine free bleaching. ECF sequences use chlorine dioxide instead of chlorine. TCF, or totally chlorine free, bleaching doesn't use any chlorine containing chemicals for bleaching.
What are the different configurations for filtrate recycling?
Fresh water could be used for washing after every stage, but filtrate is recycled to minimize water usage. The main configurations for filtrate recycling are direct countercurrent, jump-stage, split-flow, and fractional.
Below is a transcript of the video sample provided for this module:
One way to wash pulp in the bleach plant would be to supply fresh water to each stage, but this would require a tremendous amount of water. Note that as the pulp moves through the plant, there is less lignin to remove, hence, less dissolved lignin. So the pulp and wash filtrate becomes increasingly clean because the discharge filtrate from the last stage is cleaner than the pulp coming into the previous stage, that filtrate can be used as a wash water in the previous stage. Using this technique of filtrate recycling, wash water flows countercurrent to the pulp through a bleach plant. However, direct countercurrent flow is not ideal because of the pH differences from stage to stage. A large amount of acid or alkali would be required to raise and lower the pulp pH.
Use the additional resources and links below to learn more about this topic: